A TUNER by G. Nudd Home Having always felt a need for a tuner with continuous notes to replace the old tuning fork which had to be struck, I finally got round to building one. Basing the design on the frequency accuracy requirement: << O,1% E.O.M. No.74, it was decided to use three TTL 74191 ICs, programmed with a diode matrix for the base frequencies, controlled by the 12 position 'note switch'. Circuit. It was also felt that a square wave output was somewhat difficult to use for tuning purposes, so switched octave filters had to be built in as well. The low pass octave filters used here do not give a perfect sine wave but give a good approximation, and are switched with the 7 position 'octave switch'. Filter circuit. I already had a 10MHz Xtal unit with TTL output so a simple doubler was added on the end to obtain the required 20MHz input frequency. Oscillator circuit. [Alternative osc. cct.] The whole unit was built in a small box with an amplifier and speaker as well, all running from a single +5 volt stabilised (LM 309 K) supply and has proved quite useful for tuning and checking organs, guitars etc. An audio output was also provided for using with other amplifiers. A point to watch for - the capacitors in the filters must be good high frequency types as one or two filters didn't work at first with some poorer types. 74191 ICs are used, being particularly good for this application, allowing easy cascading with the max/min outputs which are a full clock period wide (not just a clock pulse wide, which is very narrow at the 2nd and 3rd ICs). A speed up system is also used to allow the circuit to work at 20MHz (the circuit functioned to 27MHz), which consists of allowing the counter to count down from the programmed number to the number two (binary 0010) at which point a flip flop (the 7473) is set, which, on the next clock pulse, resets and loads the counter, allowing a full clock period for this operation. The current consumption of the unit can be reduced by using TTL LS (low power Schottky) chips instead of the ordinary ones. The transistors in the filter unit should be high gain audio types eg NPN - BC 109, BCY 59 X etc, PNP - BFX 65, BCY 79 X etc. etc. The ICs are as seen from above, as all the circuits were built on one side of test boards (the other side all copper for earth purposes - important at high frequencies). The note switch goes from Eb up to D rather than C# - C because this allows us to use less diodes as the most significant binary digit is then always one so this input is simply connected to +5 volts. The note switch wiring only carries D.C., and not audio or other signals, so is completely uncritical. The range of the unit is from 4699 Hz down to 39 Hz. Some further points to aid understanding of the circuits: The output pulse of the 74191 dividers is very narrow (1 in 2000) and therefore useless as audio. The signal therefore passes a following divide by two (the other half of the 7473) which gives a 1:1 square wave output which being devoid of even harmonics makes the task of filtering easier. The 7493 ICs divide the signal by 2, seven times, giving the lower octaves. The octaves are selected by the 7400 switches and then reduced in amplitude by the following two resistors as the filters cannot handle 5 volt signals. The filters are normal Sallen and Key low pass, selected so that the two capacitors are as correct as possible (one 4 times the other) and the turnover frequencies at the top of each octave (ie just above the D frequency). The diode matrix was built as follows: the vertical lines were on printed board and the horizontal lines were long thick wire bridges soldered to the board at their ends with the wire a few millimeters above the board. It is then relatively easy to solder the diodes to the board at one end and to the wire at the other. FREQUENCIES PRODUCED:
20,000,640 Hz
Divided by 2x FREQUENCIES Hz
the following: PRODUCED CORRECT MAX.ERROR
2128 4699.4 4698.6 +0.016%
2255 4434.7 4434.9
2389 4186.0 4186.0
2531 3951.1 3951.1
2682 3728.7 3729.3 -0.017%
2841 3520.0 3520.0
3010 3322.4 3322.4 -0.014%
3189 3135.9 3136.0 +0.012%
3379 2959.6 2960.0
3578 2794.2 2793.8
3792 2637.2 2637.0
4018 2488.9 2489.0
DIODE MATRIX:
DIVISOR BREAKDOWN BINARY
256 16 1
2128 8 4 0 1 000 0101 0000
2255 8 12 15 1 000 1100 1111
2389 9 5 5 1 001 0101 0101
2531 9 14 3 1 001 1110 0011
2682 10 7 10 1 010 0111 1010
2841 11 1 9 1 011 0001 1001
3010 11 12 2 1 011 1100 0010
3189 12 7 5 1 100 0111 0101
3379 13 3 3 1 101 0011 001l
3578 13 15 11 1 101 1111 1011
3792 14 13 0 1 110 1101 0000
4018 15 11 2 1 111 1011 0010
--------------------ooooo00000ooooo--------------------E.O.C.S. - Electronic Organ Constructors Society |
